Electrical tester for non-metallic linings



c. w. FlGLlO ETAL 2,812,491

ELECTRICAL. TESTER FOR NON-METALLIC LININGS 2 Sheets-Sheet 1 Nov. 5, 1957 Original Filed March 8, 1951 0 W U W INVENTORS 'ZQWL JW ATTO R N EY Nov. 5, 1957 c. w. FlGLlO ETAL 2,812,491

ELECTRICAL TESTER FOR NON-METALLIC LININGS Original Filed March 8, 1951 2 Sheets-Sheet 2 PLATE VOLT'5 camp VOLT5 ATTORN EY a 2,812,491 6 Patented Nov. 5,1957

3 Claims. 1 (Cl. 324-54 This invention is directed to a rapid method of detecting breaks in non-metallic coatings, by means of an apparatus herein described. More specifically, this invention consists of a method for measuring the electrical resistance of anon-conducting protective coating with such accuracy that a break in the coating maybe detected at a considerable distance from that break. This application is a continuation of our copending application, Serial No. 214,585, filed March 8, 1951, now abandoned.

--In the chemical industry where the handling of corrosive liquids has always been a problem, it has long been the custom to coat vessels that are to be used in chemical processes with linings that are impervious to corrosive liquids. The coating materials may vary widely, dependingupon the use to which they will be subjected, one of the. mostresistant coating materials being glass. Larger vessels for use in the field, having a capacity up to 100,000 gallons, are coated with elastomers, synthetic resins or enamels.

The life of such chemical equipment is limited by the resistance of the protective coating applied, and it is not uncommon for these coatings to develop small cracks or breaks after extended use. If these breaks are discovered shortly after they occur, it is usually possible to repair the coating at that point, whereby an impervious film may be maintained indefinitely by periodic inspection and minor repairs. If periodic inspections are not made, the container may become so corroded that the entire piece of equipment must be replaced.

One of the earliest methods of detecting small breaks in a non-conducting protective coating was reported by Walker and Hickson of the National Bureau of Standards about 1929. Their method employed a galvanometer connection in series with a low-voltage battery, a resistor and an electrode. One pole of the battery was grounded to the metallic vessel and the electrode (which consisted of a. wick saturated with a conducting electrolyte) was passed over the surface to be tested. The disadvantage of this testing procedure was that the electrode had to make a direct contactwith the opening in the protective coating to obtain a reading on the galvanometer. It became a very tedious operation to cover the entire surface of a large vessel with the probe of this instrument, and unless direct contact was made at the exact point of the break, it was very easy to overlook the opening until irreparable damage had occurred.

. A modification of the Walker method which retained all the disadvantages of th'atmethod, was the so-called phenolphthalein method which utilized a sodium chloride solution containing a few drops of phenolphthalein as the Y indicator in place of a galvanometer. When the circuit was closed'at a break in the coating, electrolysis of the sodium chloride produced caustic soda and gave a red coloration to the electrolyte. In large tanks, testing became a two-man operation, one man maneuvering the electrode within the dark tank, the second man observing the electrolyte outside of the tank." In order to obtain more rapid reading by the phenolphthalein method, the operator was encouraged to increase the battery voltage which, if combustible vapors were present, introduced the additional' hazard of an explosion.

A third method of testing chemical vessels is the socalled dielectric method which has found a great deal of favor in the testing of glass-lined equipment. A difference in potential is applied between the surface. of the coating to be tested and the conducting vessel, with the result that a spark discharge immediately appears at any break. Although the dielectric method may be used conveniently in the testing of new equipment before it leaves the factory, it does not lend itself to use in the field because of its power requirements, nor may it be used in the vicinity of explosive mixtures. A further disadvantage of'the dielectric method is apparent where the coating tested has a low break-down potential. In many cases, the voltage applied is sufficiently high to break down and destroy a thin coating where no actual break is present.

In' our new method of testing for breaks, we employ ail-oscillator, the frequency of which is preferably within the audible range and-controlled by an RC circuit. Associated with our oscillator is an audio detector and means forplacing theresistance of the coating to be tested in series or parallel with the RC circuit.

It is well known that when a condenser and a resistor are connected in series with a source of E. M. R, the initial flow of current into the condenser is limited by the resistance, so that a longer period of time is required to charge the condenser than otherwise would be the case without the resistor. to. the capacity and resistance, the product of which is called the time constant of the circuit. Thus, any change in the resistance of a coating (such as would be caused by a break) will vary the time constant of the resistance capacity? combination which determines the frequency of the oscillator. It is this change in frequency which we utilize in our method of testing to locate defects.

The apparatus ofthe present invention may be used to test any non-conductor lining applied to conducting surfaces. It may, for instance, evenbe used to test a rubberlined wooden tub bygthe simple expedient of introducing a conductor such as water, between the surface of the tub and thelining. Our apparatus has the advantage that it will not break down lining that is sound.-

The chief advantage of this instrument, however, is.

that for; the first time one is able to detect a break in the lining at a distance of. 1 to 2 feet and determine the exact locationvery quickly. This speeds up the testing procedure because it is no longer necessary to test every point on the surface'of the coating. This instrument is particularly valuable in detecting any break in a coat- :Fig.'3. illustrates a so-called negative resistance oscillator.

Fig 4 illustrates a circuit employing a 'thyratron tube. 'Fig. Silhistrates the-relation between grid voltage and plate voltage of the thyratron tube. r

'Fig. 6 illustrates a multivibrator which maybe used asthe oscillators."v e

In our. preferred embodiment of this invention, we'use 1' a glow-tube. in; the. manner illustrated by thewiring idiaa The time required is proportional or harm in any way a.

3. granr'of Fig. 1; In this'diagram, battery B consists of two-45--volt-B- batteries" conneeted-inseriesfor atotal output of 90 volts. The negative pole of the battery is connected to the binding ,postG, and the positive pole is Connected to oneside of a-24 ,000" ohm current liiniting resistor-R1. Si is a SPST switchand Sis-a 3circuit selector' switch: Rh"is*'a 500'megohm potentiometer-Jud C1, Czand C3 are condensershavihga cpacity of '033','0.5 and :7 mfdz; "respectively? The indicatorconsistsofla l watt*2l5- volt neon glovw'lamp. Theimetalib'ase andcarbon resistorbuilf'into this'lamp aretremoved andthe lamp is, mountedin' a special socket; l isa closed circuit phone jack that permitsthe. use ofphones if'an audible signalgis desired.

The: entire. instrument maybe; enclosed-in. a case that caneasily'be carriedinonehandf The neon glow tube, phone jack; binding ppsts G and Sandth'e externalcontrolsSi, .Szfand'Rtz are mounted'on. one exterior. surface. of tliecase. Thisinstrument is operatedas followse. The

binding postide'ntifi'ed'by the letter G" is 'groundedfto the condiicting'surfa'ce of the tank to be .tested'andia swab. which-may. consistxofab'sorbent cotton, is saturated. with an'electrolyte'. This electrolyte may be. made.up.of'500 partsa'of water; 500ipartsalcohol, 10 parts. ofi'35.%. livd'rochloricacid and 25 parts of a surface active. agent. such as. di-octyl sodium sulfosuccinate. The. selector. switch S2 is then positionediso that. Ci-isin. parallel withthe glow tube and the swab is connectedby a: copper wire...

unbroken surface. As the swab nears a break in the coat ing, the short flashes will increase in frequency; thismay occur as far as two feet from the surface break, but it may always be. detected when theswab approaches within one foot of the surface break. The. closer. the swab approaches the break, the more frequently the lamp flashes, untila pulsating glow is. obtainedwhen theswab is directly on, the. break.

Under normal conditions, the instrument is used with S2 in such position that the 0.3 mfd. condenser istinzparalleL withthe glow tube. increasethe capacity. of this condenser inlorder that the instrument. may beused under varying atmospheric conditi'onss On a .cool, damp day it: may be foundthatzthe lampwill flash very rapidly, even though the coating itself. may be perfectly sound. If S2 is then changed so. that. a largercondenser' is in the.circuit, the flashing' frequency of theneonbulb maybe reduced: so that the.instrumentwilloperate exactly thesame as it does on a. warm, dry day.

If an audible signal is desired by the operator, apair of phones may be placed in series. with'the. circuit with out the addition of an amplifier stage.

The oscillator described. above may be. conveniently used in the dark interiorof large tanks, as its frequency corresponds exactly with. the flashing of the glow tube. It is sturdy, simple in construction, light in weightv and mobile. It will not initiatean explosion and'may be easily handled by one man. Furthermore, the operating condition. of,v the instrument is. always known, as dead batteries. or any. other defect: would-.be indicated by failure of the neon tube to flash.

The relative arrangementof. the circuit. components of Figure 1 is flexible. For example the batteries may be isolated aseparate container. Under such. circumstaneeqhowever, thatportionof the-battery on the high Provision. ismade, however, to

. lator of Fig. 1.

voltage side of the current limiting should be well insulated'from'ground to avoid all possibility of sparking. Fig. 2 illustrates a method of connecting the swab in parallel with the resistance which controls the oscillator frequency.

Any of the oscillators that are well known in the electronics; art maybe substituted for, the relaxation oscil- Frequently, the oscillator period (t) is an inverse function of the resistance, but the so-called. negative resistance oscillator of Fig. 3 may be readily adapted to. our invention. We do not wish to limit this invention to any particular type of oscillator, as any oscillator the frequency of which is a single value function of'resistance may be employed.

The diagram of a circuit that employs a thyratrontube (such as the 884) is shown in Fig. 4. To understand the operation of this circuit, assume that thethyratron used is a type 885 which has the grid characteristics shown in Fig. 4. Using again a volt B battery Eb, reference to :Fig. 5 will indicate that the grid bias Ea should be about 7 /2 volts. If the condenser C is initially uncharged and the.:swab S is grounded to. G, the voltage across. the. condenser will increase exponentiallyaccording to the equation where .t;is thetime inseconds, R is the resistance in megohms and C is-thecapacity inmicrofarads. This charg ing processwhich occurs in the RC circuit, will continue. until the voltage across the condenser reaches. approxis... mately 60 volts, at which time thetube will break down. Thecharge will leak off the condenservery rapidly and: thetube will stop conducting when the cathode to anode potential. falls. below about 16 volts, the maintaining voltage of the 885 tube. The condenser will then recharge through the RC circuit and the entire process will repeat'itself. The period of oscillation will bedependent upon R, Cand the voltages Ec andEa.

The multivibrator may be used as the oscillator ofourinvention acording to Fig. 6. The values of R3 and R4 and B should preferably be so chosen that" the instrument; operates Within the audible range.

Our preferred detector when employing'any-of the circuits discussed above; is a pair of earphones. It will be recognized, however, thatthe natural period of the. oscil lator may be above the audio frequency range and stillbe operativefor' the purpose of this invention. It is only necessary toadda second beat-frequency oscillator which will produce a heterodyne in the audible. range. If} one wishes to use a loud speaker, as the pulse detector, an amplifierstage may be added.

We claim:

1. A rapid-method of detecting. minute breaksin the non-conducting lining of metallic chemical vesselsprotected by said non-conducting liningfrom corrosive attack which comprises applying a. low voltage source between the said metal kettle and a parallel neon glow tube condenser circuit through a variable resistance, saidparallel circuit being in series with a swab, saturated with a conducting electrolyte, said swab being. very small in area in comparison with the total area of the saidnon-conducting lining being tested, and moving the said swab, while detecting the rate of flashing of the neon glow tube, over the area of said non-conducting lining, and determining the location of saidbreaks in said non-conducting lining before the said, swab has contacted said breaks from the increase in the rate of flashing of said neon glow tube when moved over a distance many times. the diameter of 5 6 of said neon glow tube is detected by sound through 21 2,170,487 Soler Aug. 22, 1939 closed audi'o circuit in series with the swab. 2,376,232 Cummings May 15, 1945 2,379,947 Bandur July 10, 1945 References Cited in the file of this patent 2,504,848 Kunz Apr. 18, 1950 5 2,515,736 Saville July 18, 1950 1938 684 Z Z STATES PATENTSD 12 1933 2,572,597 Connor ..2 Oct. 23, 1951 on ec. 2,031,840 McCarty Feb. 25, 1936 FOREIGN PATENTS 2,105,542 Martensson Jan, 18, 1938 494,345 Great Britain Oct. 25, 1938 

1. A RAPID METHOD OF DETECTING MINUTE BREAKS IN THE ON-CONDUCTING LINING OF METALLIC CHEMICAL VESSELS PROTECTED BY SAID NON-CONDUCTING LINING FROM CORROSIVE ATTACK WHICH COMPRISES APPLYING A LOW VOLTAGE SOURCE BETWEEN THE SAID METAL KETTLE AND A PARALLEL NEON GLOW TUBE CONDENSER CIRCUIT THROUG A VARIABLE RESISTANCE, SAID PARALLEL CIRCUIT BEING IN SERIES WITH S SWAB SATURATED WITH A CONDUCTING ELECTROLYTE, SAID SWAB BEING VERY SMALL IN AREA IN COMPARISON WITH THE TOTAL AREA OF THE SAID NON-CONDUCTING LINING BEING TESTED, AND MOVING THE SAID SWAB, WHILE DETACTING THE RATE OF FLASHING OF THE NEON GLOW TUBE, OVER THE AREA OF SAID NON-CONDUCTING LINING, AND DETERMINING THE LOCATION OF SAID BREAKS IN SAID NON-CONDUCTING LINING BEFORE THE SAID SWAB HAS CONTACTED SAID NEON GLOW TUBE INCREASE IN THE RATE OF FLASHING OF SID NEON GLOW TUBE WHEN MOVED OVER A DISTANCE MANY TIMES THE DIAMETER OF SAID SWAB FROM A PLURALITY OF DIRECTION RELATIVE TO SAID BREAM, THUS OBTAINING A CROSS-BEARING ON THE LOCATION OF SAID BREAK. 